It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone", the presence of the marker is detected and an alarm is generated. Some EAS markers are intended to be removed at the checkout counter upon payment for the merchandise. Other types of markers remain attached to the merchandise but are deactivated upon checkout by a deactivation device which changes a characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
An EAS system has been proposed which includes an application of the so-called "giant magneto-impedance" (GMI) effect. The GMI effect is a phenomenon in which the voltage induced by a high frequency current source in a ferromagnetic wire is substantially changed by applying an external DC magnetic field to the wire.
An EAS system according to this proposal is somewhat schematically illustrated in FIGS. 1 and 2. The system shown in FIGS. 1 and 2 includes pedestals 10 and 11, disposed on opposite sides of a doorway 12. The pedestals are arranged to provide an alarm signal whenever a marker 13 attached to a garment 14 is brought within range, provided, of course, that the marker 13 is in an activated condition.
The marker, to be described hereinafter, includes a wire (not shown in FIGS. 1 and 2) which exhibits the abovementioned GMI effect. One or both of the pedestals include respective antennas which transmit into an interrogation zone at the doorway 12 a microwave carrier signal, and a relatively low frequency alternating magnetic field. The active wire component of the marker 13 is preferably cut to a length equal to half the wavelength of the microwave carrier signal. The wire is therefore able to efficiently receive and re-emit the microwave energy. The low frequency magnetic field, if incident along the length of the wire, modulates the effective impedance of the wire at the frequency of the magnetic field signal. This produces a side band signal of the microwave carrier frequency. The resulting signal which is radiated from the marker is quite unique, and can be readily detected by a suitable receiver included in one or both of the pedestals. The interaction between the marker 13 and the pedestals 10, 11 is schematically illustrated in FIG. 2, in which the block captioned "surveillance system" represents the pedestals 10, 11 and the electronic circuitry incorporated therein.
Although the doorway 12 shown in FIG. 1 is relatively narrow, it is believed that an EAS system utilizing the microwave-GMI marker referred to above may operate effectively to cover an interrogation zone having a width of several meters or more.
It could be contemplated to provide a deactivable microwave-GMI marker, for use with the EAS system illustrated in FIGS. 1 and 2, according to a construction which is schematically illustrated in FIG. 3. Element 20 shown in FIG. 3 is the above-mentioned GMI wire, cut to the half-wavelength of the microwave carrier of the EAS system. Deactivation elements 22 are positioned at intervals along the wire 20. (Those of ordinary skill will recognize that the deactivation element configuration shown in FIG. 3 is similar to that employed in a deactivable harmonic-type EAS marker like that shown in U.S. Pat. No. 5,341,125.) As would be expected by those who are skilled in the art, the deactivation elements 22 would be formed of a material having semi-hard ferromagnetic properties.
When it is desired to deactivate the marker, a DC magnetic field would be applied along the length of the wire 20 at a level sufficiently high to magnetize the deactivation elements 22. The resulting bias magnetic fields applied by the deactivation elements 22 to the wire 20 interferes with the GNI effect that would otherwise be caused by the low frequency magnetic interrogation field, so that the sideband modulation of the marker signal does not take place, and the marker is not detectable by the surveillance system 15. However, as deactivation would be carried out in practice in a retail store using conventional deactivation devices, it may be difficult or impossible to assure that the deactivation field to be applied to the deactivation elements 22 is oriented along the length of the wire 20. As the inventors of the present invention have recognized, any misalignment of the deactivation field relative to the length of the wire may fail to magnetize the deactivation elements 22 in such a way that they substantially interfere with the GNI effect. Consequently, a marker having the configuration shown in FIG. 3 is likely not to be reliably deactivated by known practices.